Electrochemical Production of Urea from NOx and Carbon Dioxide

ABSTRACT

Methods and systems for electrochemical production of urea are disclosed. A method may include, but is not limited to, steps (A) to (B). Step (A) may introduce carbon dioxide and NOx to a solution of an electrolyte and a heterocyclic catalyst in an electrochemical cell. The divided electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode may reduce the carbon dioxide and the NOx into a first sub-product and a second sub-product, respectively. Step (B) may combine the first sub-product and the second sub-product to produce urea.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit under 35 U.S.C. §120 of U.S. patent application Ser. Number 12/875,227, naming Narayanappa Sivasankar, Emily Cole, Kyle Teamey, and Andrew Bocarsly as inventors, filed Sep. 3, 2010, now pending, which in turn claims the benefit under 35 U.S.C. §120 of the following applications:

U.S. patent application Ser. No. 12/846,221, entitled REDUCING CARBON DIOXIDE TO PRODUCTS, naming Emily Cole, Narayanappa Sivasankar, Andrew Bocarsly, Kyle Teamey, and Nety Krishna as inventors, now pending, filed Jul. 29, 2010.

U.S. patent application Ser. No. 12/845,995, entitled PURIFICATION OF CARBON DIOXIDE FROM A MIXTURE OF GASES, naming Kyle Teamey, Emily Cole, Narayanappa Sivasankar, and Andrew Bocarsly as inventors, now pending, filed Jul. 29, 2010.

U.S. patent application Ser. No. 12/846,011, entitled HETEROCYCLE CATALYZED ELECTROCHEMICAL PROCESS, naming Emily Cole and Andrew Bocarsly as inventors, now pending, filed Jul. 29, 2010.

U.S. patent application Ser. No. 12/846,002, entitled ELECTROCHEMICAL PRODUCTION OF SYNTHESIS GAS FROM CARBON DIOXIDE, naming Narayanappa Sivasankar, Emily Cole, and Kyle Teamey as inventors, now pending, filed Jul. 29, 2010.

Each of the above-listed applications is hereby incorporated by reference in their entireties.

FIELD

The present disclosure generally relates to the field of chemical reduction, and more particularly to a method and/or apparatus for implementing electrochemical production of urea from NOx and carbon dioxide.

BACKGROUND

The combustion of fossil fuels in activities such as electricity generation, transportation, and manufacturing produces billions of tons of carbon dioxide annually. Research since the 1970s indicates increasing concentrations of carbon dioxide in the atmosphere may be responsible for is altering the Earth's climate, changing the pH of the ocean and other potentially damaging effects. Countries around the world, including the United States, are seeking ways to mitigate emissions of carbon dioxide.

A mechanism for mitigating emissions is to convert carbon dioxide into economically valuable materials such as fuels and industrial chemicals.

If the carbon dioxide is converted using energy from renewable sources, both mitigation of carbon dioxide emissions and conversion of renewable energy into a chemical form that can be stored for later use will be possible. Urea is an important fertilizer and industrial chemical used around the world. Industrially, urea is synthesized from carbon dioxide and ammonia at temperatures between 150 to 210 degrees Celsius and pressures of 120 to 400 atmospheres. Ammonia is typically produced from hydrogen and nitrogen at relatively high temperatures and pressures. The overall process of industrially synthesizing urea requires a large amount of energy, which generally comes from natural gas. The combustion of natural gas contributes to the concentration of carbon dioxide in the atmosphere and thus, global climate change.

Previous work in the field of electrochemical techniques has many limitations, including the stability of systems used in the process, the efficiency of systems, the selectivity of the systems or processes for a desired chemical, the cost of materials used in systems/processes, the ability to control the processes effectively, and the rate at which carbon dioxide is converted. In particular, existing electrochemical and photochemical processes/systems have one or more of the following problems that prevent commercialization on a large scale. Several processes utilize metals such as ruthenium or gold that are rare and expensive. In other processes, organic solvents were used that made is scaling the process difficult because of the costs and availability of the solvents, such as dimethyl sulfoxide, acetonitrile and propylene carbonate. Copper, silver and gold have been found to reduce carbon dioxide to various products, however, the electrodes are quickly “poisoned” by undesirable reactions on the electrode and often cease to work in less than an hour. Similarly, gallium-based semiconductors reduce carbon dioxide, but rapidly dissolve in water. Many cathodes produce a mixture of organic products. For instance, copper produces a mixture of gases and liquids including carbon monoxide, methane, formic acid, ethylene, and ethanol. Such mixtures of products make extraction and purification of the products costly and can result in undesirable waste products that must be disposed. Much of the work done to date on carbon dioxide reduction is inefficient because of high electrical potentials utilized, low faradaic yields of desired products, and/or high pressure operation. The energy consumed for reducing carbon dioxide thus becomes prohibitive. Many conventional carbon dioxide reduction techniques have very low rates of reaction. For example, in order to provide economic feasibility, a commercial system currently may require densities in excess of 100 milliamperes per centimeter squared (mA/cm2), while rates achieved in the laboratory are orders of magnitude less.

SUMMARY

A method for electrochemical production of urea may include, but is not limited to, steps (A) to (B). Step (A) may introduce carbon dioxide and NOx to a solution of an electrolyte and a heterocyclic catalyst in an electrochemical cell. The electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode may reduce the carbon dioxide to a first sub-product and the NOx to a second sub-product. Step (B) may combine the first sub-product and the second sub-product to produce urea.

A method for electrochemical production of urea may include, but is not limited to, steps (A)-(C). Step (A) may introduce carbon dioxide and NOx to a solution of an electrolyte and a heterocyclic catalyst in an electrochemical cell. The electrochemical cell may include an anode in a first cell compartment and a cathode in a second cell compartment. The cathode may reduce the carbon dioxide to a first sub-product and the NOx to a second sub-product. Step (B) may combine the first sub-product and the second sub-product to produce urea. Step (C) may vary a yield of urea by adjusting at least one of (a) a cathode material, (b) the heterocyclic catalyst type, (c) an electrical potential of the cathode, and (d) the electrolyte type.

A system for electrochemical production of urea may include, but is not limited to, an electrochemical cell, a carbon dioxide source, a NOx source, and an energy source. The electrochemical cell may include a first cell compartment, an anode positioned within the first cell compartment, a second cell compartment, a separator interposed between the first cell compartment and the second cell compartment, and a cathode and a heterocyclic catalyst positioned within the second cell compartment. The first cell compartment and the second cell compartment may each contain an electrolyte. The carbon dioxide source may be coupled with the second cell compartment and be configured to supply carbon dioxide to the cathode. The NOx source may be coupled with the second cell compartment and be configured to supply NOx to the cathode. The fluid source may be coupled with the first cell compartment. The energy source may be operably coupled with the anode and the cathode and be configured to provide power to the anode and the cathode, to reduce carbon dioxide at the cathode to a first sub-product, to reduce NOx at the cathode to a second sub-product, and to oxidize the fluid at the anode. The first sub-product and the second sub-product may be configured to combine to form urea.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the disclosure as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the disclosure and together with the general description, serve to explain the principles of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the present disclosure may be better understood by those skilled in the art by reference to the accompanying figures in which:

FIG. 1 is a block diagram of a system in accordance with an embodiment of the present disclosure;

FIG. 2 is a table illustrating relative product yields for different cathode materials, catalysts, and electrolyte combinations;

FIG. 3 is a formula of an aromatic heterocyclic amine catalyst;

FIGS. 4-6 are formulae of substituted or unsubstituted aromatic 5-member heterocyclic amines or 6-member heterocyclic amines;

FIG. 7 is a flow diagram of an example method used in electrochemical examples; and

FIG. 8 is a flow diagram of an example method used in photochemical examples.

DETAILED DESCRIPTION

Reference will now be made in detail to the presently preferred embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings.

In accordance with some embodiments of the present invention, an electrochemical system is provided that generally allows carbon dioxide and NOx to be converted to urea. In some embodiments, the energy used by the system may be generated from an alternative energy source to avoid generation of additional carbon dioxide through combustion of fossil fuels.

The reduction of carbon dioxide may be suitably catalyzed by heterocyclic catalysts which may include nitrogen, sulfur, and oxygen-containing heterocycles and substituted heterocycles (e.g., pyridine, imidazole and substituted derivatives). The system may include electrolytes consisting of water as a solvent and suitable salts that are water soluble.

Some embodiments of the present invention thus relate to environmentally beneficial methods for reducing carbon dioxide. The methods generally include electrochemically and/or photoelectrochemically reducing the carbon dioxide in an aqueous, electrolyte-supported divided electrochemical cell that includes an anode in a cell compartment and a cathode in another cell compartment. A catalyst may be included to produce a reduced product. Carbon dioxide may be continuously bubbled is through the cathode electrolyte solution to saturate the solution.

For electrochemical reductions, the electrode may be a suitable conductive electrode, such as Al, Au, Ag, C, Cd, Co, Cr, Cu, Cu alloys (e.g., brass and bronze), Ga, Hg, In, Mo, Nb, Ni, Ni alloys, Ni-Fe alloys, Sn, Sn alloys, Ti, V, W, Zn, stainless steel (SS), austenitic steel, ferritic steel, duplex steel, martensitic steel, Nichrome, elgiloy (e.g., Co-Ni-Cr), degenerately doped p-Si, degenerately doped p-Si:As and degenerately doped p-Si:B. Other conductive electrodes may be implemented to meet the criteria of a particular application. For photoelectrochemical reductions, the electrode may be a p-type semiconductor, such as p-GaAs, p-GaP, p-InN, p-InP, p-CdTe, p-GaInP₂ and p-Si. Other semiconductor electrodes may be implemented to meet the criteria of a particular application.

Control of the electrochemical process may enable production of a desired product by controlling combinations of metal cathodes, catalysts, and electrolytes. Efficiency of the process may be selectively increased by employing a catalyst/cathode combination selective for reduction of carbon dioxide to a first sub-product (e.g., carbon monoxide or other reduced CO₂ species such as surface bound —HCOO and —HCO moieties) in conjunction with cathode materials selective for reducing NOx to a second sub-product (e.g., ammonia or an ammonia-like compound (i.e., an intermediate product resulting from the reduction of NOx such as surface bound —NHO species)). For catalytic reduction of carbon dioxide, the cathode materials may include Sn, Ag, Cu, steel, and alloys of Cu and Ni. For catalytic reduction of NOx, the cathode materials may include Ni, Pt, and Au.

The catalyst for conversion of carbon dioxide and NOx electrochemically or photoelectrochemically may be nitrogen, sulfur, and oxygen-containing heterocycles which may include pyridine, imidazole, pyrrole, thiazole, furan, and thiophene. The catalyst may also include substituted heterocycles, such as amino-thiazole and benzimidazole. A heterocyclic amine catalyst may be utilized which may include, but is not limited to, heterocyclic compounds that are 5-member or 6-member rings with at least one ring nitrogen. For example, pyridines, imidazoles and related species with at least one five-member ring, bipyridines (e.g., two connected pyridines) and substituted derivatives were generally found suitable as catalysts for the electrochemical reduction and/or the photoelectrochemical reduction. Amines that have sulfur or oxygen in the rings may also be suitable for the reductions. Amines with sulfur or oxygen may include thiazoles or oxazoles. Other aromatic amines (e.g., quinolines, adenine, azoles, indoles, benzimidazole and 1,10-phenanthroline) may also be effective electrocatalysts.

Carbon dioxide may be photochemically or electrochemically reduced to carbon monoxide or other reduced CO₂ intermediates, and NOx may be photochemically or electrochemically reduced to ammonia or an ammonia-like intermediate compound. The carbon monoxide and the ammonia or ammonia-like compound may combine to form urea as a product of the system. Current reduction processes are generally highly energy-consuming and thus are not efficient ways for a high yield, economical conversion of carbon dioxide and NOx to urea.

On the other hand, the use of processes for converting carbon dioxide and NOx to urea in accordance with some embodiments of the invention generally has the potential to lead to a significant reduction of carbon dioxide, a major greenhouse gas, in the atmosphere and thus to the is mitigation of global warming.

Before any embodiments of the invention are explained in detail, it is to be understood that the embodiments may not be limited in application per the details of the structure or the function as set forth in the following descriptions or illustrated in the figures of the drawing. Different embodiments may be capable of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of terms such as “including,” “comprising,” or “having” and variations thereof herein are generally meant to encompass the item listed thereafter and equivalents thereof as well as additional items. Further, unless otherwise noted, technical terms may be used according to conventional usage.

In the following description of methods, process steps may be carried out over a range of temperatures (e.g., approximately 1° C. (Celsius) to 70° C.) and a range of pressures (e.g., approximately 1 to 10 atmospheres) unless otherwise specified. Numerical ranges recited herein generally include all values from the lower value to the upper value (e.g., all possible combinations of numerical values between the lowest value and the highest value enumerated are considered expressly stated). For example, if a concentration range or beneficial effect range is stated as 1% to 50%, it is intended that values such as 2% to 40%, 10% to 30%, or 1% to 3%, etc., are expressly enumerated. The above may be simple examples of what is specifically intended.

A use of electrochemical or photoelectrochemical reduction of carbon dioxide and NOx, tailored with certain electrocatalysts, may produce urea in a high yield of approximately 80% to 100% as a relative percentage of carbon-containing products, based on the amount of carbon dioxide. The yield may suitably be about 90% to 100%, and more suitably about 95% to 100%. With an electric potential of −0.5 to −1.4 volts (V) with respect to a saturated calomel electrode (SCE), urea may be produced with good faradaic efficiency at the cathode.

The reduction of the carbon dioxide and NOx may be suitably achieved efficiently in a divided electrochemical or photoelectrochemical cell in which (i) a compartment contains an anode suitable to oxidize or split the water, and (ii) another compartment contains a working cathode electrode and a catalyst. The compartments may be separated by a porous glass frit, microporous separator, ion exchange membrane, or other ion conducting bridge. Both compartments generally contain an aqueous solution of an electrolyte. Carbon dioxide gas may be continuously bubbled through the cathodic electrolyte solution to saturate the solution.

In the working electrode compartment, carbon dioxide may be continuously bubbled through the solution. In some embodiments, if the working electrode is a conductor, an external bias may be impressed across the cell such that the potential of the working electrode is held constant. In other embodiments, if the working electrode is a p-type semiconductor, the electrode may be suitably illuminated with light. An energy of the light may be matching or greater than a bandgap of the semiconductor during the electrolysis. Furthermore, either no external source of electrical energy may be used or a modest bias (e.g., about 500 millivolts) may be applied. The working electrode potential is generally held constant relative to the SCE. The electrical energy for the electrochemical reduction of carbon dioxide may come from a normal energy source, including nuclear and alternatives (e.g., hydroelectric, wind, solar power, geothermal, etc.), from a solar cell, or other nonfossil fuel source of electricity, provided that the electrical source supply at least approximately 1.5 volts across the cell. Other voltage values may be adjusted depending on the internal resistance of the cell employed.

Advantageously, the carbon dioxide may be obtained from any source (e.g., an exhaust stream from fossil-fuel burning power or industrial plants, from geothermal or natural gas wells or the atmosphere itself). Most suitably, the carbon dioxide may be obtained from concentrated point sources of generation prior to being released into the atmosphere. For example, high concentration carbon dioxide sources may frequently accompany natural gas in amounts of 5% to 50%, exist in flue gases of fossil fuel (e.g., coal, natural gas, oil, etc.) burning power plants, and nearly pure carbon dioxide may be exhausted from cement factories and from fermenters used for industrial fermentation of ethanol. Certain geothermal steams may also contain significant amounts of carbon dioxide. The carbon dioxide emissions from varied industries, including geothermal wells, may be captured on-site. Separation of the carbon dioxide from such exhausts is known. Thus, the capture and use of existing atmospheric carbon dioxide in accordance with some embodiments of the present invention generally allow the carbon dioxide to be a renewable and unlimited source of carbon.

The electrochemical/photoelectrochemical reduction of the carbon dioxide generally utilizes one or more catalysts in the aqueous solution. Aromatic heterocyclic amines may include, but are not limited to, unsubstituted and substituted pyridines and imidazoles. Substituted pyridines and imidazoles may include, but are not limited to mono and disubstituted pyridines and imidazoles. For example, suitable catalysts may include straight chain or branched chain lower alkyl (e.g., C1-C10) mono and disubstituted such as 2-methylpyridine, 4-tertbutyl pyridine, 2,6-dimethylpyridine (2,6-lutidine); bipyridines, such as 4,4′-bipyridine; amino-substituted pyridines, such as 4-dimethylamino pyridine; and hydroxyl-substituted pyridines (e.g., 4-hydroxy-pyridine) and substituted or unsubstituted quinoline or isoquinolines. The catalysts may also suitably include substituted or unsubstituted dinitrogen heterocyclic amines, such as pyrazine, pyridazine and pyrimidine. Other catalysts generally include azoles, imidazoles, indoles, oxazoles, thiazoles, substituted species and complex multi-ring amines such as adenine, pterin, pteridine, benzimidazole, phenonthroline and the like.

Referring to FIG. 1, a block diagram of a system 100 is shown in accordance with a specific embodiment of the present invention. The system (or apparatus) 100 generally comprises a cell (or container) 102, a liquid source 104, a power source 106, a gas source 108, an extractor 110 and an extractor 112. A product may be presented from the extractor 110. An output gas may be presented from the extractor 112. Another output gas may be presented from the cell 102.

The cell 102 may be implemented as a divided cell. The divided cell maybe a divided electrochemical cell and/or a divided photochemical cell. The cell 102 is generally operational to reduce carbon dioxide (CO₂) and nitrogen oxides (NOx, which may be nitrites and/or nitrates) into urea. The reduction generally takes place by bubbling carbon dioxide and NOx into an aqueous solution of an electrolyte in the cell 102. A cathode 120 in the cell 102 may reduce the carbon dioxide and the NOx into one or more compounds. The one or more compounds formed from the reduction of the carbon dioxide and the NOx may combine to form urea as a product.

The cell 102 generally comprises two or more compartments (or chambers) 114 a-114 b, a separator (or membrane) 116, an anode 118, and a cathode 120. The anode 118 may be disposed in a given compartment (e.g., 114 a). The cathode 120 may be disposed in another compartment (e.g., 114 b) on an opposite side of the separator 116 as the anode 118. An aqueous solution 122 may fill both compartments 114 a-114 b. The aqueous solution 122 may include water as a solvent and water soluble salts (e.g., potassium chloride (KCl) and potassium nitrite (KNO2)). A catalyst 124 may be added to the compartment 114 b containing the cathode 120.

The liquid source 104 may implement a water source. The liquid source 104 may be operational to provide pure water to the cell 102.

The power source 106 may implement a variable voltage source. The power source 106 may be operational to generate an electrical potential between the anode 118 and the cathode 120. The electrical potential may be a DC voltage.

The gas source 108 may implement a carbon dioxide source and a NOx source. The source 108 is generally operational to provide carbon dioxide and NOx to the cell 102. In some embodiments, the carbon dioxide and/or the NOx is bubbled directly into the compartment 114 b containing the cathode 120.

The extractor 110 may implement an organic product and/or inorganic product extractor. The extractor 110 is generally operational to extract (separate) products (e.g., urea) from the electrolyte 122. The extracted products may be presented through a port 126 of the system 100 for subsequent storage and/or consumption by other devices and/or processes.

The extractor 112 may implement an oxygen extractor. The extractor 112 is generally operational to extract oxygen (e.g., O₂) byproducts created by the reduction of the carbon dioxide and/or the oxidation of water. The extracted oxygen may be presented through a port 128 of the system 100 for subsequent storage and/or consumption by other devices and/or processes. Chlorine and/or oxidatively evolved chemicals may also be byproducts in some configurations, such as in an embodiment of processes other than oxygen evolution occurring at the anode 118. Such processes may include chlorine evolution, oxidation of organics, and corrosion of a sacrificial anode. Any other excess gases (e.g., hydrogen) created by the reduction of the carbon dioxide and water may be vented from the cell 102 via a port 130.

In the process described, water may be oxidized (or split) to protons and oxygen at the anode 118 while the carbon dioxide is reduced to carbon monoxide or a CO₂-derived intermediate species at the cathode 120 and the NOx is reduced to ammonia or an ammonia-like intermediate compound at the cathode 120. The electrolyte 122 in the cell 102 may use water as a solvent with any salts that are water soluble and with a pyridine or pyridine-derived catalyst 124. The catalysts 124 may include, but are not limited to, nitrogen, sulfur and oxygen containing heterocycles. Examples of the heterocyclic compounds may be pyridine, imidazole, pyrrole, thiazole, furan, thiophene and the substituted heterocycles such as amino-thiazole and benzimidazole. Cathode materials generally include any conductor. However, efficiency of the process may be selectively increased by employing a catalyst/cathode combination selective for reduction of carbon dioxide to carbon monoxide or a reduced CO₂ intermediate species in conjunction with cathode materials selective for reducing NOx to ammonia or an ammonia-like intermediate compound. For catalytic reduction of carbon dioxide, the cathode materials may include Sn, Ag, Cu, steel, and alloys of Cu and Ni. For catalytic reduction of NOx, the cathode materials may include Ni, Pt, and Au. The materials may be in bulk form. Additionally and/or alternatively, the materials may be present as particles or nanoparticles loaded onto a substrate, such as graphite, carbon fiber, or other conductor.

An anode material sufficient to oxidize or split water may be used. The overall process may be generally driven by the power source 106. Combinations of cathodes 120, electrolytes 122, and catalysts 124 may be used to control the reaction products of the cell 102.

Experiments were conducted in one, two, and three-compartment electrochemical cells 102 with an SCE as the reference electrode. A platinum anode or mixed metal oxide anodes were utilized. The experiments were generally conducted at ambient temperature and pressure. Carbon dioxide was bubbled into the cells during the experiments. NOx was introduced to the electrolyte of the cell. A potentiostat or DC power source 106 provided the electrical energy to drive the process. Cell potentials ranged from 2 volts to 4 volts, depending on the cathode material. Half cell potentials at the cathode ranged from −0.5 volts to −1.45 volts relative to the SCE, depending on the cathode material used. Products from the experiments were analyzed using gas chromatography, nuclear magnetic resonance spectroscopy, and a quadrupole mass spectrometer.

Referring to FIG. 2, a table illustrating relative product yields for varying cathode material, catalyst, electrolyte, and cathode potential combinations are shown. The combinations listed in the tables generally are not the only combinations providing a given product. The combinations illustrated demonstrate yields of the products as relative percentages of carbon-containing products observed. As shown in FIG. 2, a stainless steel cathode (SS 316) with a 30 mM imidazole catalyst with a cathode potential of −1.4 (versus SCE) yielded 100% urea (relative to organic products). In the instance where no catalyst was used, with copper as the cathode material, no urea or acetone was produced, thus demonstrating the importance of the catalyst in producing urea. In the instance where an imidazole catalyst was used with copper as the cathode material, 1% urea was produced, with the balance 99% as acetone.

Faradaic yields for the products may be improved by controlling the electrical potential of the reaction. By maintaining a constant potential at the cathode 120, hydrogen evolution is generally reduced and faradaic yields of the products increased. Addition of hydrogen inhibitors such as acetonitrile, certain heterocycles, alcohols, and other chemicals may also increase yields of the products.

With some embodiments, stability may be improved with cathode materials known to poison rapidly when reducing carbon dioxide. Copper and copper-alloy electrodes commonly poison in less than an hour of electrochemically reducing carbon dioxide. However, when used with a heterocyclic amine catalyst, copper-based alloys were operated for many hours without any observed degradation in effectiveness. The effects were particularly enhanced by using sulfur containing heterocycles. For instance, a system with a copper cathode and 2-amino thiazole catalyst showed very high stability for the reduction of carbon dioxide.

Heterocycles other than pyridine may catalytically reduce carbon dioxide in the electrochemical process using many aforementioned cathode materials, including tin, steels, nickel alloys and copper alloys. Nitrogen-containing heterocyclic amines shown to be effective include azoles, indoles, 4,4′-bipyridines, picolines (methyl pyridines), lutidines (dimethyl pyridines), hydroxy pyridines, imidazole, benzimidazole, methyl imidazole, pyrazine, pyrimidine, pyridazine, pyridazineimidazole, nicotinic acid, quinoline, adenine and 1,10-phenanthroline. Sulfur containing heterocycles include thiazole, aminothiazoles, thiophene. Oxygen containing heterocycles include furan and oxazole. As with pyridine, the combination of catalyst, cathode material and electrolyte may be used to control product mix.

Referring to FIG. 3, a formula of an aromatic heterocyclic amine catalyst is shown. The ring structure may be an aromatic 5-member heterocyclic ring or 6-member heterocyclic ring with at least one ring nitrogen and is optionally substituted at one or more ring positions other than nitrogen with R. L may be C or N. R1 may be H. R2 may be H if L is N or R2 is R if L is C. R is an optional substitutent on any ring carbon and may be independently selected from H, a straight chain or branched chain lower alkyl, hydroxyl, amino, pyridyl, or two R's taken together with the ring carbons bonded thereto are a fused six-member aryl ring and n=0 to 4.

Referring to FIGS. 4-6, formulae of substituted or unsubstituted aromatic 5-member heterocyclic amines or 6-member heterocyclic amines are shown. Referring to FIG. 4, R3 may be H. R4, R5, R7 and R8 are generally independently H, straight chain or branched chain lower alkyl, hydroxyl, amino, or taken together are a fused six-member aryl ring. R6 may be H, straight chain or branched chain lower alkyl, hydroxyl, amino or pyridyl.

Referring to FIG. 5, one of L1, L2 and L3 may be N, while the other L's may be C. R9 may be H. If L1 is N, R10 may be H. If L2 is N, R11 may be H. If L3 is N, R12 may be H. If L1, L2 or L3 is C, then R10, R11, R12, R13 and R14 may be independently selected from straight chain or branched chain lower alkyl, hydroxyl, amino, or pyridyl.

Referring to FIGS. 6, R15 and R16 may be H. R17, R18 and R19 are generally independently selected from straight chain or branched chain lower alkyl, hydroxyl, amino, or pyridyl.

Suitably, the concentration of aromatic heterocyclic amine catalysts is about 1 millimolar (mM) to 1 M. The electrolyte may be suitably a salt, such as KCl, NaNO₃, Na₂SO₄, NaCl, NaF, NaClO₄, KClO₄, K₂SiO₃, or CaCl₂ at a concentration of about 0.5 M. Other electrolytes may include, but are not limited to, all group 1 cations (e.g., H, Li, Na, K, Rb and Cs) except Francium (Fr), Ca, ammonium cations, alkylammonium cations and alkyl amines. Additional electrolytes may include, but are not limited to, all group 17 anions (e.g., F, Cl, Br, I and At), borates, carbonates, nitrates, nitrites, perchlorates, phosphates, polyphosphates, silicates and sulfates. Na generally performs as well as K with regard to best practices, so NaCl may be exchanged with KCl. NaF may perform about as well as NaCl, so NaF may be exchanged for NaCl or KCl in many cases. The pH of the solution is generally maintained at about pH 3 to 8, suitably about 4.7 to 5.6.

Some embodiments of the present invention may be further explained by the following examples, which should not be construed by way of limiting the scope of the invention.

EXAMPLE 1 General Electrochemical Methods

Chemicals and materials. All chemicals used were >98% purity and used as received from the vendor (e.g., Aldrich), without further purification. Either deionized or high purity water (Nanopure, Barnstead) was used to prepare the aqueous electrolyte solutions.

Electrochemical system. The electrochemical system was composed of a standard two-compartment electrolysis cell 102 to separate the anode 118 and cathode 120 reactions. The compartments were separated by a porous glass frit or other ion conducting bridge 116. The electrolytes 122 were used at concentrations of 0.1 M to 1 M, with 0.5 M being a typical concentration. A concentration of between about 1 mM to 1 M of the catalysts 124 was used. The particular electrolyte 122 and particular catalyst 124 of each given test were generally selected based upon what product or products were being created.

Referring to FIG. 7, a flow diagram of an example method 140 used in the electrochemical examples is shown. The method (or process) 140 generally comprises a step (or block) 142, a step (or block) 144, a step (or block) 146, a step (or block) 148 and a step (or block) 150. The method 140 may be implemented using the system 100.

In the step 142, the electrodes 118 and 120 may be activated where appropriate. Introducing the carbon dioxide and the NOx into the cell 102 may be performed in the step 144. Electrolysis of the carbon dioxide and NOx into organic and/or inorganic products may occur during step 146. In the step 148, the products may be separated from the electrolyte. Analysis of the reduction products may be performed in the step 150.

The working electrode was of a known area. All potentials were measured with respect to a saturated calomel reference electrode (Accumet). Before and during all electrolysis, carbon dioxide (Airgas) was continuously bubbled through the electrolyte to saturate the solution. The resulting pH of the solution was maintained at about pH 3 to pH 8 with a suitable range depending on what product or products were being made.

For example, under constant carbon dioxide bubbling, the pH levels of 10 mM solutions of 4-hydroxy pyridine, pyridine and 4-tertbutyl pyridine were 4.7, 5.28 and 5.55, respectively.

EXAMPLE 2 General Photoelectrochemical Methods

Chemicals and materials. All chemicals used were analytical grade or higher. Either deionized or high purity water (Nanopure, Barnstead) was used to prepare the aqueous electrolyte solutions.

Photoelectrochemical system. The photoelectrochemical system was composed of a Pyrex three-necked flask containing 0.5 M KCl as supporting electrolyte and a 1 mM to 1 M catalyst (e.g., 10 mM pyridine or pyridine derivative). The photocathode was a single crystal p-type semiconductor etched for approximately 1 to 2 minutes in a bath of concentrated HNO3:HCl, 2:1 v/v prior to use. An ohmic contact was made to the back of the freshly etched crystal using an indium/zinc (2 wt. % Zn) solder. The contact was connected to an external lead with conducting silver epoxy (Epoxy Technology H31) covered in glass tubing and insulated using an epoxy cement (Loctite 0151 Hysol) to expose only the front face of the semiconductor to solution. All potentials were referenced against a saturated calomel electrode (Accumet). The three electrode assembly was completed with a carbon rod counter electrode to minimize the reoxidation of reduced carbon dioxide products. During all electrolysis, carbon dioxide gas (Airgas) was continuously bubbled through the electrolyte to saturate the solution. The resulting pH of the solution was maintained at about pH 3 to 8 (e.g., pH 5.2).

Referring to FIG. 8, a flow diagram of an example method 160 used in the photochemical examples is shown. The method (or process) 160 generally comprises a step (or block) 162, a step (or block) 164, a step (or block) 166, a step (or block) 168 and a step (or block) 170. The method 160 may be implemented using the system 100.

In the step 162, the photoelectrode may be activated. Introducing the carbon dioxide and the NOx into the cell 102 may be performed in the step 164. Electrolysis of the carbon dioxide and NOx into the products may occur during step 166. In the step 168, the products may be separated from the electrolyte. Analysis of the reduction products may be performed in the step 170.

Light sources. Four different light sources were used for the illumination of the p-type semiconductor electrode. For initial electrolysis experiments, a Hg-Xe arc lamp (USHIO UXM 200H) was used in a lamp housing (PTI Model A-1010) and powered by a PTI LTS-200 power supply. Similarly, a Xe arc lamp (USHIO UXL 151H) was used in the same housing in conjunction with a PTI monochromator to illuminate the electrode at various specific wavelengths.

A fiber optic spectrometer (Ocean Optics 52000) or a silicon photodetector (Newport 818-SL silicon detector) was used to measure the relative resulting power emitted through the monochromator. The flatband potential was obtained by measurements of the open circuit photovoltage during various irradiation intensities using the 200 watt (W) Hg-Xe lamp (3 W/cm2-23 W/cm2). The photovoltage was observed to saturate at intensities above approximately 6 W/cm2.

For quantum yield determinations, electrolysis was performed under illumination by two different light-emitting diodes (LEDs). A blue LED (Luxeon V Dental Blue, Future Electronics) with a luminous output of 500 milliwatt (mW) +/−50 mW at 465 nanometers (nm) and a 20 nm full width at half maximum (FWHM) was driven at to a maximum rated current of 700 mA using a Xitanium Driver (Advance Transformer Company). A Fraen collimating lens (Future Electronics) was used to direct the output light. The resultant power density that reached the window of the photoelectrochemical cell was determined to be 42 mW/cm2, measured using a Scientech 364 thermopile power meter and silicon photodetector.

The measured power density was assumed to be greater than the actual power density observed at the semiconductor face due to luminous intensity loss through the solution layer between the wall of the photoelectrochemical cell and the electrode.

EXAMPLE 3 Analysis of Products of Electrolysis

Electrochemical experiments were generally performed using a CH Instruments potentiostat or a DC power supply with current logger to run bulk electrolysis experiments. The CH Instruments potentiostat was generally used for cyclic voltammetry. Electrolysis was run under potentiostatic conditions from approximately 1 hour to 30 hours until a relatively similar amount of charge was passed for each run.

Gas Chromatography. The electrolysis samples were analyzed using a gas chromatograph (HP 5890 GC) equipped with a FID detector. Removal of the supporting electrolyte salt was first achieved with an Amberlite IRN-150 ion exchange resin (cleaned prior to use to ensure no organic artifacts by stirring in a 0.1% v/v aqueous solution of Triton X-100, reduced (Aldrich), filtered and rinsed with a copious amount of water, and vacuum dried below the maximum temperature of the resin (approximately 60° C.) before the sample was directly injected into the GC which housed a DB-Wax column (Agilent Technologies, 60 m, 1 micrometer (pm) film thickness). Approximately 1 gram of resin was used to remove the salt from 1 milliliter (mL) of the sample. The injector temperature was held at 200° C., the oven temperature maintained at 120° C., and the detector temperature at 200° C.

Mass spectrometry. Mass spectral data was also collected to identify all organic compounds. In a typical experiment, the sample was directly leaked into a SRS Quadrupole Mass Spectrometer.

Nuclear Magnetic Resonance. NMR spectra of electrolyte volumes after bulk electrolysis were also obtained using an automated Bruker Ultrashield™ 500 Plus spectrometer with an excitation sculpting pulse technique for water suppression. Data processing was achieved using MestReNova software. The concentrations of urea and acetone present after bulk electrolysis were determined using acetonitrile or imidazole as the internal standards. NMR was the primary means of determining urea concentrations, showing a singlet peak at 5.6 ppm.

Carbon dioxide and NOx may be efficiently converted to value-added products, using either a minimum of electricity (that may be generated from an alternate energy source) or directly using visible light. Some processes described above may generate urea useful for chemical processes. Moreover, the catalysts for the processes may be substituents-sensitive and provide for selectivity of the value-added products.

By way of example, a fixed cathode (e.g., stainless steel 2205) may be used in an electrochemical system where the electrolyte and/or catalyst are altered to change the product mix. In a modular electrochemical system, the cathodes may be swapped out with different materials to change the product mix. In a hybrid photoelectrochemical system, the anode may use different photovoltaic materials to change the product mix.

Some embodiments of the present invention generally provide for new cathode materials, new electrolyte materials and new sulfur and oxygen-containing heterocyclic catalysts. Specific combinations of cathode materials, electrolytes, catalysts, and/or electrical potentials may be used to get a desired product. The organic products may include, but are not limited to, urea. Specific process conditions may be established that maximize the carbon dioxide and NOx conversion to specific chemicals beyond urea.

Cell parameters may be selected to minimize unproductive side reactions like H2 evolution from water electrolysis. Choice of specific configurations of heterocyclic amine pyridine catalysts with engineered functional groups may be utilized in the system 100 to achieve high faradaic yields. Process conditions described above may facilitate long life (e.g., improved stability), electrode and cell cycling and product recovery. Heterocyclic amines related to pyridine may be used to improve reaction rates, product yields, cell voltages and/or other aspects of the reaction. Heterocyclic catalysts that contain sulfur or oxygen may also be utilized in the carbon dioxide and NOx reduction.

Some embodiments of the present invention may provide cathode and electrolyte combinations for reducing carbon dioxide to products in commercial quantities. Catalytic reduction of carbon dioxide may be achieved using steel or other low cost cathodes. High faradaic yields (e.g., >20%) of organic products with steel and nickel alloy cathodes at ambient temperature and pressure may also be achieved. Copper-based alloys used at the electrodes may remain stable for long-term reduction of carbon dioxide. The relative low cost and abundance of the combinations described above generally opens the possibility of commercialization of electrochemical carbon dioxide reduction.

Various process conditions disclosed above, including cathode materials, electrolyte choice, catalyst choice, and cell voltage, generally improve control of the reaction so that different products or product mixtures may be made. Greater control over the reaction generally opens the possibility for commercial systems that are modular and adaptable to make different products. The new materials and process conditions combinations generally have high faradaic efficiency and relatively low cell potentials, which allows an energy efficient cell to be constructed.

It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the disclosure or without sacrificing all of its material advantages. The form herein before described being merely an explanatory embodiment thereof, it is the intention of the following claims to encompass and include such changes. 

1-17. (canceled)
 18. A system for electrochemical production of urea, comprising: an electrochemical cell including: a first cell compartment; an anode positioned within the first cell compartment; a second cell compartment; a separator interposed between the first cell compartment and the second cell compartment, the first cell compartment and the second cell compartment each containing an electrolyte; and a cathode and a heterocyclic catalyst positioned within the second cell compartment; a carbon dioxide source, the carbon dioxide source coupled with the second cell compartment, the carbon dioxide source configured to supply carbon dioxide to the cathode; a NOx source, the NOx source coupled with the second cell compartment, the NOx source configured to supply NOx to the cathode; a fluid source, the fluid source coupled with the first cell compartment; and an energy source operably coupled with the anode and the cathode, the energy source configured to provide power to the anode and the cathode, to reduce carbon dioxide at the cathode to a first sub-product, to reduce NOx at the cathode to a second sub-product, and to oxidize the fluid at the anode, the product mixture configured to combine to form urea.
 19. The system of claim 18, wherein the cathode includes a stainless steel 316 material.
 20. The system of claim 18, wherein a yield of urea is between approximately 90% and 100% relative to a total of carbon-containing products. 